1. Field of the Invention
The present invention relates to a soft-start circuit, and more particularly to a soft-start circuit for charging a current source by using a current difference.
2. Description of Related Art
In general, a power supply device will output a large current to an output terminal of the power supply device while being just started, and such a large current may damage a load or components coupled to the load. To avoid the aforementioned damage when the power supply device is just started, the power supply device controls the output energy at the beginning of a startup to rise gradually until the power supply device works normally, so as to prevent possible damage to circuits coupled to the power supply device, and this type of arrangement is called a soft start.
A pulse width modulation controller achieves the soft-start effect by increasing the working cycle gradually from the beginning, and thus the power supply device includes a voltage source that is increased with time and used for a soft-start control.
With reference to FIG. 1A for a schematic circuit diagram of a conventional soft-start circuit, the soft-start circuit includes a current mirror 10, a P-type bipolar transistor 15, a soft-start capacitor Cl and a reset switch 20. The current mirror 10 is coupled to a working power source VDD, a bias voltage source Vbias, and the P-type bipolar transistor 15, for supplying a current Ie (which is an emitter current) to the P-type bipolar transistor 15. The current Ie flowing through the P-type bipolar transistor 15 is divided into a base current Ib and a collector current Ic, wherein Ib/Ie is equal to 1/(1+β), and β is the current gain of the P-type bipolar transistor 15. The soft-start capacitor C1 is coupled to a base of the P-type bipolar transistor 15, and the base current Ib is used for performing a charging to supply a soft-start voltage SS that increases with time. The reset switch 20 receives a soft-start control signal XEN, such that when the circuit is started, the soft-start control signal XEN is pulled low to turn off the reset switch 20, and the soft-start voltage SS rises with time. If the circuit needs to be started again, the soft-start control signal XEN will be pulled high to discharge electric charges stored in the soft-start capacitor C1 for the use of a next soft-start.
Since the β value of the bipolar transistor increases with a rise of temperature and decreases with a drop of temperature, the soft-start time of the soft-start circuit as shown in FIG. 1A is affected significantly by ambient temperature. With reference to FIG. 1B for a simulated voltage curve of a soft-start voltage SS of a soft-start circuit as depicted in FIG. 1A and situated at different temperatures, the difference among time points of t1, t2, t3, the soft-start voltage SS reaching a soft-start termination voltage Se at temperatures T1, T2, T3 is very large. In the simulation, the difference of the soft-start time between a high temperature of 125° C. and a low temperature of −40° C. is approximately three times, and thus the error of the soft-start time of a soft-start circuit as shown in FIG. 1A does not fall into an acceptable range.
With the same soft-start time requirement, a smaller current source can be used for reducing the size of a charging capacitor to facilitate building the capacitor into an integrated circuit. With reference to FIG. 2A for a schematic circuit diagram of another conventional soft-start circuit, the soft-start circuit includes a current mirror 10, a Darlington circuit 25, a soft-start capacitor C2, and a reset switch 20. Compared with the P-type bipolar transistor 15 of the circuit as shown in FIG. 1A, the current Ib′ outputted from the Darlington circuit 25 is equal to 1/+β)2 times of the Ie provided by the current mirror 10, and thus the size of the soft-start capacitor C2 can be reduced to 1/(1+β) times of the soft-start capacitor C1 as shown in FIG. 1A. However, the Darlington circuit 25 also amplifies the effect of the ambient temperature. With reference to FIG. 2B for a simulated voltage curve of a soft-start voltage SS of a soft-start circuit as depicted in FIG. 2A and situated at different temperatures, the simulation is conducted at temperatures T1, T2 (and current may leak at the temperature T3 since the charging current is too small, so that the Darlington circuit 25 cannot be started at this temperature in the simulation), and the soft-start voltage SS reaches the soft-start termination voltage Se at the time points of t4, t5 respectively. According to the aforementioned charging current formula, the variation of the charging current of the Darlington circuit 25 with temperature is the square of that of a single bipolar transistor, Thus, the soft-start circuit as shown in FIG. 2A varies with temperature to a greater extent, and the error of the soft-start time still does not fall within an acceptable range.